1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to protective wiring devices.
2. Technical Background
Electrical distribution systems as defined herein, are systems configured to provide power to structures such as residences, commercial buildings or other such facilities. Such systems typically include one or more breaker panels coupled to a source of AC power. A breaker panel distributes AC power to one or more branch electric circuits installed in the structure. The electric circuits may typically include one or more receptacle outlets and may further transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. Receptacle outlets provide power to user-accessible, or portable, loads. Loads of this type are connected to a power cord and plug. As everyone knows, user-accessible loads obtain power by inserting the plug into the receptacle outlet.
Certain types of fault conditions have been known to occur in various portions of the electrical distribution systems. System designers have responded to these fault conditions by employing electric circuit protection devices in strategic positions throughout the distribution system, such as in the breaker panel and in protective devices (having receptacle outlets) disposed in the various branches of the distribution system. Protective devices may also be installed in the electrical load itself.
Electrical wiring devices as well as protective wiring devices are typically disposed in an electrically non-conductive housing. The housing provides access to electrical terminals that are electrically insulated from each other. As those skilled in the art understand, line terminals are employed to couple the wiring device to an electrical power source. Load terminals are coupled to wiring that directs AC power to one or more electrical loads disposed in the branch circuit. Load terminals may also be referred to as “feed-through” terminals because the wires connected to these terminals may be coupled to a daisy-chained configuration of receptacles or switches. The load may ultimately be connected at the far end of this arrangement. The load terminals may also be connected to an electrically conductive path that is also connected to a set of receptacle contacts. The receptacle contacts are in communication with receptacle openings disposed on the face of the housing. This arrangement allows a user to insert an appliance plug into the receptacle opening to thereby energize the device. Those of ordinary skill in the pertinent art will understand that the term “load” refers to an appliance, a switch, or some other electrically powered device.
There are several types of electric circuit protection devices that may be used depending on device location and device function. For example, such devices include ground fault circuit interrupters (GFCIs), ground-fault equipment protectors (GFEPs), Transient voltage surge suppressors (TVSSs) and arc fault circuit interrupters (AFCIs). Some devices include both GFCIs and AFCIs. This list includes representative examples and is not meant to be exhaustive.
As their names suggest, arc fault circuit interrupters (AFCIs), ground-fault equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs) perform different functions. An arc fault typically manifests itself as a high frequency current signal. Accordingly, an AFCI may be configured to detect various high frequency signals and de-energize the electrical circuit in response thereto.
A ground fault occurs when a current carrying (hot) conductor creates an unintended current path to ground. A differential current is created between the hot/neutral conductors because some of the current flowing in the circuit is diverted into the unintended current path. The unintended current path represents an electrical shock hazard. Ground faults, as well as arc faults, may also result in fire or represent a fire hazard. A “grounded neutral” is another type of ground fault. This type of fault may occur when the load neutral terminal, or a conductor connected to the load neutral terminal, becomes grounded.
Transient voltage surge suppressors (TVSSs) are designed to protect the branch circuit from lightning storms and from switched loads that impart transient over-voltages on the electrical distribution system. Some devices include both a TVSS and some other type of protective device. When a device is installed, its line terminals are connected to an AC power source, such as a single phase 120 VAC AC power source. Transient voltages may propagate in both the electrical distribution system as well as the AC power source. The amplitudes of transient voltages are typically greater than the amplitude of the source voltage by at least an order of magnitude. Transient voltage pulses may be generated by any number of events. For example, transient voltages may be introduced into the distribution system by lightning. Transient voltages may also be generated when an inductive load is turned off, when a motor with noisy brushes is operated, or by other events.
Transient voltages are known to damage protective devices such that the device ceases to function as designed. This is sometimes referred to as an end of life condition. When an end of life condition occurs in a GFCI, end of life failure modes include failure of device circuitry, the relay solenoid that opens the GFCI interrupting contacts, and/or failure of the solenoid driving device, such as a silicon controlled rectifier (SCR).
In some failure modes, the aforementioned damage may result in the protective device permanently denying power to the protected portion of the electric circuit. In this case, the user must replace the protective device to restore power to the protected portion of the circuit. In other failure modes, the damage may result in the protective device still providing power to the load even though the device has become non-protective and the user is left unprotected after an end-of-life condition has occurred. In either case, the user is either inconvenienced by having to change out the device, or even worse, is left unprotected.
To protect the device against damaging transient voltages, most devices are equipped with surge protection components. However, surge protection components occupy a considerable volume within the device housing. One drawback to surge protection components relates to their size, making the overall size of the device relatively large. Of course, relatively large devices are more difficult to install in a wall box because of the available space constraints. Another problem is that surge protective components themselves are known to experience an end-of-life condition. If the surge protection component fails, the device is unprotected from transient voltage damage and the device may become a shock hazard.
In general, a spark gap is often used to protect sensitive electrical or electronic equipment from high voltage surges. A spark gap typically consists of two conductive elements separated by a gas, which is usually air. During an abnormal voltage surge, the spark gap is designed to break down and safely shunt the voltage surge to ground to thereby protect the circuit from damage. The temperature of the arc occurring in the spark gap during a transient voltage condition can be greater than 1000° C. The spark gap may fail under such conditions.
Spark gap failure may occur when a component is over-heated because of its composition and close proximity to the spark gap structure. The component may be a non-electrically conductive barrier made out of plastic, resin, fibrous material, or the like. Overheating may result in the barrier in becoming electrically conductive. The barrier may continue to be conductive even after the over-voltage condition has transpired. Overheating may also cause the barrier to deform to the extent that it is no longer able to provide electrical isolation. If the component is an electrically conductive component, such as a load current-carrying component, overheating may cause it to either melt or vaporize. This may result in the development of a new conductive path.
Another form of spark gap failure involves the plasma associated with the arc. The plasma may extend far enough to envelop a nearby conductor. Since the plasma is ionic, it may conduct current from the spark originating conductor to the aforementioned nearby conductor. The conducted electrical current may be enough to impair the operation of the protective device.
Spark gap failure may result in the protective device becoming susceptible to nuisance tripping. Like other failure modes, spark gap failure may cause the device to become non-protective. Even worse, this failure mode may result in a fire hazard or a shock hazard. Thus, non-conductive barriers and electrically conductive components must be disposed a sufficient distance from the spark gap. Heretofore this has not been possible because the size of the device enclosure is restricted by the size of the wall box. Unfortunately, components must be placed near the spark gap structure where they are vulnerable to the heat released during a transient voltage event. As an alternative, the components inside the enclosure must be arranged in an efficient and compact manner to overcome the size constraints.
Accordingly, a compact protective device having an improved space-conserving surge protection arrangement is needed. The aforementioned device must continue to provide reliable fault protection after the voltage transient event occurs. Further, a protective device is needed that is equipped to decouple the load terminals from the line terminals in the event of an end of life condition.